Differential Sensor Based on Electroinductive Wave Transmission Lines for Dielectric Constant Measurements and Defect Detection

A differential-mode sensor based on a pair of electroinductive wave transmission lines (EIW-TLs) is proposed in this article. The EIW-TLs are implemented by etching a chain of complementary split-ring resonators (CSRRs) in a metallic plate. Such metallic plate acts as the ground plane of the microstrip (access) lines, necessary for feeding each EIW-TL and collecting the transmitted power at the output ports. The working principle of the sensor is mode conversion, caused when the EIW-TLs are asymmetrically loaded. Thus, one of the lines is loaded with the reference (REF) sample, whereas the other one is loaded with the sample under test (SUT). If both samples are identical and the pair of EIW-TLs is fed by a common-mode signal, a pure common-mode signal is collected at the differential output port. Conversely, mode conversion arises when the lines are unequally loaded, due to the different propagation characteristics (particularly the phase velocity) of the lines. Due to the typical dispersion relation of the EIW-TLs, with strong variation in the phase velocity (or phase constant) with frequency, the structure is very sensitive to dielectric constant differences between the REF and SUT samples. Thus, the proposed structure is useful for the accurate measurement of dielectric constants (sensor functionality) and the detection of tiny differences between the REF and SUT samples (comparator functionality).